Is training a CNN object detector on an image containing multiple targets that are not all annotated will teach it to miss targets?
The neural network will learn what we teach it, for example with that image only, after finish training, your model will hard to recognize humans with dark skin, glasses, big eyes, etc, the features that two annotated targets don't have.
If your data is big enough, and contain all the feature of humans face, the result should be good.
If not, I recommend a semi unsupervised learning method which called Noisy Student. Quick explanation, you take a part of data, add noise (augmentation, drop-out, stochastic depth), and train. Then use that model to label the rest of the dataset and train a new bigger teacher (such as YOLOv3 &gt; YOLOv1) and repeat it. They told that this method was even better than you labeled all the dataset.
You will need to choose the data to train the teacher really carefully, then pray for the result.